This invention relates generally to improvements in the production of ozone (O.sub.3). More particularly, the invention relates to the electrolytic production of ozone utilizing a proton exchange membrane to separate the anode and depolarized cathode.
Ozone has long been recognized as a useful chemical commodity valued particularly for its outstanding oxidative activity. Because of this activity, it finds wide application in disinfection processes. In fact, it kills bacteria more rapidly than chlorine, it decomposes organic molecules, and removes coloration in aqueous systems. Ozonation removes cyanides, phenols, iron, maganese, and detergents. It controls slime formation in aqueous systems, yet maintains a high oxygen content in the system. Unlike chlorination, which may leave undesirable chlorinated organic residues in organic containing systems, ozonation leaves fewer potentially harmful residues. There is evidence that ozone will destroy viruses. It is used for sterilization in the brewing industry and for odor control in sewage treatment and manufacturing. And ozone is employed as a raw material in the manufacture of certain organic compounds, e.g., oleic acid and peroxyacetic acid.
Thus, ozone has widespread application in many diverse activities, and its use would undoubtedly expand if its cost of production could be reduced. In addition, since ozone is explosive when concentrated as either a gas or liquid, or when dissolved into solvents or absorbed into gels, its transportation is potentially hazardous. Therefore, it is generally manufactured on the site where it is used. However, the cost of generating equipment, and poor energy efficiency of production has deterred its use in many applications and in many locations.
On a commercial basis, ozone is currently produced by the silent electric discharge process, otherwise known as corona discharge, wherein air or oxygen is passed through an intense, high frequency alternating current electric field. The corona discharge process forms ozone through the following reaction: EQU 3/2O.sub.2 =O.sub.3 ; .DELTA.H.degree..sub.298 =34.1 kcal
Yields in the corona discharge process generally are in the vicinity of 2% ozone, i.e., the exit gas may be about 2% O.sub.3 by weight. Such O.sub.3 concentrations, while quite poor, in an absolute sense, are still sufficiently high to furnish usable quantities of O.sub.3 for the indicated commercial purposes. Another disadvantage of the corona process is the production of harmful NO.sub.x, otherwise known as nitrogen oxides. Other than the aforementioned electric discharge process, there is no other commercially exploited process for producing large quantities of O.sub.3.
However, O.sub.3 may also be produced by the electrolytic process, wherein an electric current (normally D.C.) is impressed across electrodes immersed in an electrolyte, i.e., electrically conducting, fluid. The electrolyte includes water, which, in the process, dissociates into its respective elemental species, O.sub.2 and H.sub.2. Under the proper conditions, the oxygen is also evolved as the O.sub.3 species. The evolution of O.sub.3 may be represented as: EQU 3H.sub.2 O=O.sub.3 +3H.sub.2 ; .DELTA.H.degree..sub.298 =207.5 kcal
It will be noted that the .DELTA.H.degree. in the electrolytic process is many times greater than that for the electric discharge process. Thus, the electrolytic process appears to be at about a six-fold disadvantage.
More specifically, to compete on an energy cost basis with the electric discharge method, an electrolytic process must yield at least a six-fold increase in ozone. Heretofore, the necessary high yields have not been realized in any foreseeably practical electrolytic system.
The evolution of O.sub.3 by electrolysis of various electrolytes has been known for well over 100 years. High yields up to 35% current efficiency have been noted in the literature. Current efficiency is a measure of ozone production relative to oxygen production for given inputs of electrical current, i.e., 35% current efficiency means that under the conditions stated, the O.sub.2 /O.sub. 3 gases evolved at the anode are comprised of 35% O.sub.3 by volume. However, such yields could only be achieved utilizing very low electrolyte temperatures, e.g., in the range from about -30.degree. C. to about -65.degree. C. Maintaining the necessary low temperatures, obviously requires costly refrigeration equipment as well as the attendant additional energy cost of operation.
Ozone, O.sub.3, is present in large quantities in the upper atmosphere in the earth to protect the earth from the suns harmful ultraviolet rays. In addition, ozone has been used in various chemical processes, is known to be a strong oxidant, having an oxidation potential of 2.07 volts. This potential makes it the fourth strongest oxidizing chemical known.
Because ozone has such a strong oxidation potential, it has a very short half-life. For example, ozone which has been solubilized in waste water may decompose in a matter of 20 minutes. Ozone can decompose into secondary oxidants such as highly reactive hydroxyl (OH*) and peroxyl (HO.sub.2 *) radicals. These radicals are among the most reactive oxidizing species known. They undergo fast, non-selective, free radical reactions with dissolved compounds. Hydroxyl radicals have an oxidation potential of 2.8 volts (V), which is higher than most chemical oxidizing species including O.sub.3. Most of the OH* radicals are produced in chain reactions where OH itself or HO.sub.2 * act as initiators.
Hydroxyl radicals act on organic contaminants either by hydrogen abstraction or by hydrogen addition to a double bond, the resulting radicals disproportionate or combine with each other forming many types of intermediates which react further to produce peroxides, aldehydes and hydrogen peroxide.
Electrochemical cells in which a chemical reaction is forced by added electrical energy are called electrolytic cells. Central to the operation of any cell is the occurrence of oxidation and reduction reactions which produce or consume electrons. These reactions take place at electrode/solution interfaces, where the electrodes must be good electronic conductors. In operation, a cell is connected to an external load or to an external voltage source, and electric charge is transferred by electrons between the anode and the cathode through the external circuit. To complete the electric circuit through the cell, an additional mechanism must exist for internal charge transfer. This is provided by one or more electrolytes, which support charge transfer by ionic conduction. Electrolytes must be poor electronic conductors to prevent internal short circuiting of the cell.
The simplest electrochemical cell consists of at least two electrodes and one or more electrolytes. The electrode at which the electron producing oxidation reaction occurs is the anode. The electrode at which an electron consuming reduction reaction occurs is called the cathode. The direction of the electron flow in the external circuit is always from anode to cathode.
A typical electrochemical cell will have a positively charged anode and a negatively charged cathode. The anode and cathode are typically submerged in a liquid electrolytic solution which may be comprised of water and certain salts, acids or base materials. Generally speaking, gaseous oxygen is released at the anode surface while gaseous hydrogen is released at the cathode surface. A catalyst such as lead dioxide may be used to coat the anode to get greater ozone production. The anode substrate may be another material such as titanium, graphite, or the like.
The cathode and anode are positioned within the electrolytic cell with electrical leads leading to the exterior. The cell is also provided with appropriate plumbing and external structures to permit circulation of the electrolyte to a separate heat exchanger. Suitable inlet and outlet passages are also provided in the cell head space to permit the withdrawal of the gases evolved from the cathode (if gases are to be evolved) and from the anode. The two gas removal systems are typically maintained separate in order to isolate the cathode gases from the anode gases. Nitrogen and/or air may be pumped through the gas handling system in order to entrain the evolved cathode and anode gases and carry them from the cell to the exterior where they may be utilized in the desired application. Alternately, if a flow-through air or oxygen cathode is employed, its excess gases may be used for this purpose.
In order to maintain or cool the cell electrodes, heat exchange passages may be provided within the electrode structures. These coolant passages are connected to external sources of coolant liquid which can be circulated through the electrodes during the electrolysis process in order to maintain or reduce their temperatures.
In order to drive the electrolysis reaction, it is necessary to apply electric power to the cell electrodes. The electrodes are connected through the electrical leads to an external source of electric power with the polarity being selected to induce the electrolyte anion flow to the anode and the cation flow to the cathode. The power requirements are not appreciably different for those cells utilizing platinum anodes from those cells utilizing lead dioxide anodes. Electrical potentials on the order of from 2-3 volts D.C. are quite sufficient for the various cell configurations. The current requirements are most easily measured in terms of current density and may vary from a low of perhaps a tenth of an ampere per square centimeter (0.1 A/cm.sup.2) up to current densities slightly beyond one ampere per square centimeter (.ltoreq.1.0 A/cm.sup.2). The power requirements are not necessarily dependent upon the electrolyte concentrations, nor in particular upon the anode materials. Thus, current densities of from about 0.1 A/cm.sup.2 to about 1.5 A/cm.sup.2 will produce maximum ozone current efficiencies at any electrolyte concentration with either beta lead dioxide anodes or platinum anodes.
U.S. Pat. No. 4,316,782 (Foller) teaches that ozone yields as high as 52% could be obtained where the electrolyte is water and either the acid or salt form of highly electronegative anions, such as hexafluoro-anions are used. Here, the term "fluoro-anions" is used to describe that family of anionic (negatively charged) species in which multiple fluorine ligands complex a central atom. Electrolysis was carried out in a range between room temperature and the freezing point of water. The preferred anode materials for use in the electrolytic cells are either platinum or lead dioxide, especially lead dioxide in the beta crystalline form. Platinum, carbon, or nickel and its alloys may be used as hydrogen-evolving cathodes. Alternatively, an air or oxygen depolarized cathode may be employed which would greatly reduce the cell voltage and enhance the overall energy efficiency of the process.
Such electrolytic solutions can be highly corrosive to the cell materials if they are not selected properly, and especially hard on the electrodes where electrochemical discharge takes place. In addition, the liberated O.sub.3, being a powerful oxidizing agent, also strongly acts upon electrode materials which are susceptible to oxidizing action. The electrical properties of the electrode material are also important to the successful and effective operation of the ozone generating electrolytic cell. The electrodes must exhibit sufficient electrical conductivity to enable the utilization of current densities required by the ozone generating process without an unacceptable anode potential and must also be adaptable to whatever cooling procedures are required to maintain cell temperatures during operation.
Foller also disclosed that using an air or oxygen depolarized cathode provided several advantages. (1) The cell voltage would be substantially reduced since replacing hydrogen evolution with the reduction of oxygen theoretically saves 1.23 volts. (In actual practice a 0.8 volt swing is likely to be achieved.) (2) A separator between anode and cathode is no longer required, as no hydrogen is evolved to depolarize the anode. Further, savings in cell voltage are obtained by reducing IR losses. (3) The overall cell process becomes oxygen in and ozone out and the need for periodic additions of water is reduced. (4) The same air or oxygen fed to the air cathode could also serve to dilute and carry off the ozone that is anodically evolved by flowing through the cathode.
Air cathode technology has found recent favor in its application to fuel cells, metal-air batteries, and the chlor-alkali industry. The electrodes are generally composed of Teflon-bonded carbon containing small amounts of catalytic materials.
U.S. Pat. No. 4,375,395 (Foller) teaches that anodes made of glassy carbon are suitable for use in the preparation of ozone in an electrolytic cell utilizing an aqueous solution of the highly electronegative fluoro-anions.
U.S. Pat. No. 4,541,989 (Foller) teaches that a liquid electrolyte containing acids of fluoro-anions, such as HBF.sub.4 and HPF.sub.6, used in combination with a cool electrolyte solution can increase the efficiency and the ozone to oxygen yield. However, the use of a liquid electrolyte causes some problems. First, the electrodes in such electrolytic cell must be separated by a given distance to provide definition. This translates into power loss in the production of heat. Secondly, the presence of liquid electrolytes requires a sophisticated system of seals to prevent leaking of the electrolyte.
U.S. Pat. No. 4,836,929 (Laumann et al.) teaches the use of a solid electrolyte such as that made by duPont and sold under the brand "NAFION". This solid electrolyte was placed between a lead dioxide anode and a platinum black cathode. The current efficiency was increased by oxygenating a water stream fed to the anode and the cathode. In this manner, oxygen could be reduced to water at room temperature releasing an increased yield of ozone.
In his paper entitled "Synthesis of Hydrogen Peroxide in a Proton Exchange Membrane Electrochemical Reactor" (April 1993), Fenton disclosed that paired synthesis of ozone (O.sub.3) and hydrogen peroxide (H.sub.2 O.sub.2) could be carried out in the same reactor. The electrochemical reactor used a membrane and electrode assembly (M&E) comprised of a "NAFION" 117 membrane between the platinum black/polytetrafluoroethylene (PTFE) anode and graphite/PTFE cathode. This M&E assembly was sandwiched between a carbon fiber paper (Toray Industries) on the cathode side and a platinum mesh (52 mesh, Fisher Scientific) on the anode side which were used as current collectors. This arrangement was alleged to produce some hydrogen peroxide.
Increasing the percentage of PTFE in the electrode increases the hydrophobicity of the electrode assembly and thus allows more of the gaseous reactant to reach the electrode surface by repelling the products formed. The graphite M&E with 20% PTFE produced slightly higher hydrogen peroxide than a similar M&E with 10% PTFE. This could be due to the mass transport limitation of oxygen to the membrane and electrode assembly within the less hydrophobic 10% M&E. It is preferred that the PEM reactor operate at potentials greater than 3.0 volts where the anodic evolution of ozone is favored.
Membranes containing perfluorinated sulphonic acids are typically prepared before use in an electrochemical cell by first soaking the membrane in hot water for about 30 minutes and then soaking it in 10% HCl to ensure that the entire membrane is in the H.sup.+ form. The membrane has to be kept moist at all times as it acts as a conductor only when it is wet. It is preferred that the proton exchange membrane be pretreated with an aqueous solution of sulphuric acid followed by rinsing the proton exchange membrane with pure water, rinsing the proton exchange membrane with an aqueous solution of hydrogen peroxide, and rinsing the proton exchange membrane with a final rinse of pure water. The final rinse should be made at a temperature between 50.degree. C. and 150.degree. C. and under pressure.
In their paper entitled "Paired Synthesis of Ozone and Hydrogen Peroxide in an Electrochemical Reactor," Pallav Tatapudi and James Fenton explain that the benefits of paired synthesis in electrolyte free water include: (1) lower energy consumption costs, as two oxidizing agents can be obtained for the price of one; (2) the elimination of the need for transportation and storage of oxidants by generating them electrochemically within water on demand at an amount proportional to the waste concentration; and (3) higher aqueous phase ozone concentrations.
U.S. Pat. No. 4,416,747 (Menth et al.) discloses an individual electrolysis cell bounded by bipolar plates and having a solid electrolyte made of perfluorinated sulphonic acids ("NAFION" by duPont) with a surface coating centrally located between current-collectors and adjoining open metallic structures. A plurality of individual cells may be integrated together between end plates so that the cells are electrically connected in series, hydrodynamically connected in parallel, and combined to form a block.
The current collectors disclosed in Menth may be close-meshed expanded metal covered by an open structure having a low resistance to the flow of a liquid in the direction parallel to the planar structure. The current collectors are preferably made from titanium. The ends of the cell are formed, in each case, by a bipolar plate, which alternately acts as a cathode and as an anode. The bipolar plate is preferably made from stainless (Cr/Ni) steel. The space or chamber between the bipolar plates and the solid electrolyte is completely filled with water in which air or oxygen is suspended and/or dissolved.
The Menth assembly of the electrolysis cells basically corresponds to the filter-press type, with the liquid passing parallel to the principal planes of the cells instead of perpendicularly. The individual cells are held together between two end plates having electrical terminals thereon.
The method and apparatus disclosed in Menth, however, can support only limited current density associated with reduction-oxidation since oxygen has only limited solubility in water. Further, since the cathode chamber is filled with liquid water, the cathode electrode structure will become flooded with water. Higher current densities are desirable to cause an increase in the ozone production efficiency.
U.S. Pat. No. 4,836,849 (Laumann et al.) teaches a process for breaking down organic substances and/or microbes in pretreated feed water for high-purity recirculation systems using ozone which is generated in the anode chamber of an electrochemical cell and treated with ultraviolet rays and/or with hydrogen generated in the cathode chamber of the same cell or supplied from outside.
In light of the foregoing discussion, there exists a need for an economical method of producing ozone which will minimize voltage allowed for higher current density and produce a high concentration of ozone.